During last 48 hrs my emails are deleting themselves?

2 days ago i had approx 200 or so emails in my inbox , all of a sudden they have all dissappeared & i now notice that any new emails im receiving are deleting as soon as ive read them . ive checked all my email files etc & there nothing there ..... however ive found them all still stored in my microsoft outlook email files..... plz can sum1 tell me how i can view my only email acc which is yahoo in the way i always used to by going straight to yahoo site & how do i stop them from deleting as soon as ive read them & only being saved in my microsoft outlook .. im v confused & not v computer literate so no hi tech explanations pleez !! plz email me at clairemichelle1968@yahoo.co.uk thanks michelle